Refining of aluminum



March 17, 1936. R. A. GADEAU 2,034,339

REFINING OF ALUMINUM Filed 001}. 30, 1933 Patented Mi... 17,1936

UNITED STATES PATENT orrlcz 2,034,339 REFINING or ALUMINUM ApplicationOctober 30, 1933, Serial No. 695,899

InF

rance November 8, 1932 5 Claims. (Cl. 204-20) The present inventionrelates to processes in which the aluminum to be refined alloyed with aheavy metal, copper for instance, is at the bottom of the apparatus,forming a melted anode, denser than the electrolyte, while the aluminumresulting from the electrolysis forms a liquid layer, which floats onthe electrolyte and acts as a cathode. I

It was first proposed, in processes of that kind, to utilize anelectrolyte consisting of cryolite and barium chloride.

But it was subsequently found that such a bath is not well adapted tothe electrolytic refining of aluminum, that is to say is notselective.Furthermore it was remarked that such a bath, when it contains an amountof barium chloride sufilcient for giving it a density higher than thatof aluminum, produces, at temperatures averaging 1000 C., an excessivequantity of smoke.

Finally an exclusive preference was given to baths containingalkali-earth fluorides, it being however well known that with anelectrolyte containing a mixture of cryolite and of alkali-earthfluoride there is formed at the cathode an objectionable deposit ofmetallic sodium. n order to eliminate this last mentioned drawback, thepercentage of aluminum fluoride was increased, so that the amount ofthis body present in the bath, as compared with that of sodium fluoride,was higher than in cryolite.

Now, according to the present invention, it has been found that,contrary to the indications published previously, .a bath containingaluminum fluoride, sodium fluoride, and an alkali-earth chloridepossesses all the qualities of selectivitythat are required, providedthat the amount of aluminum fluoride present therein should be such thatthe ratio of said amount and of that of so dium fluoride in the bath isgreater than that corresponding to cryolite.

Such a bath is less expensive than those containing only fluorides,since alkali-earth chlorides are cheaper than the correspondingfluorides. Furthermore, and this is a more important advantage from thetechnical point of view, it permits of working at a temperature of about700- 800 C. instead of 1000 C. with baths containing only fluorides.This possibility of working at a lower temperature eliminatessubstantially the production of smoke.

It makes it possible to utilize a tank without a water jacket, whichdoes away with the permanent dangers and the difficulties inherent inthis element. On the other hand, it very much reduces the loss byoxidation of the upper layer of refined and melted aluminum.Furthermore, due to the fact that it very much'reduces the production ofmetallic sodium, the wear and tear of the carbon cathodes through whichcurrent is fed to the layer of melted aluminum is diminished.

Of course it is necessary that the electrolyte should have a densityranging between that of aluminum and that of the alloy utilized asanode, which determines the percentage of alkali-earth chloride, thecomposition of the electrolyte being further chosen with a view togiving it a melting point as low as possible.

This is, by way of example, the composition of an electrolyte complyingwith the conditions above stated:

Per cent in weight F3A1 23 F Na 17 ClzBa .j. 0

minum fluoride, that is as follows:

4 Per cent in weight F3A1 27 F Na 13 C1238. 60

Of course the examples that have just been given are not limitative inany way and any mixture of FaAl with alkali and alkali-earth fluoridesand chlorides of a suitable density and melting point can be utilizedaccording to the present invention, provided that the ratio of FsAl andof the alkaline fluorides is greater than that corresponding to theformula of cryolite.

The process according to the present invention may be carried out invarious kinds of apparatus previously proposed for the refining ofaluminum through the known method of the three melted layers of anodicalloy, electrolyte, and cathodic aluminum.

But, as the baths according to the present invention permit working atlower temperature, and as the said baths contain a large proportion ofchlorides noncorrosive for the magnesia, it becomes possible to use atank merely coated with bricks of magnesia without any water jacket.

A preferred embodiment of an apparatus according to the presentinvention will be hereingreatly after described, with reference to theappended drawing, given merely by way of example, and in which:

The only-figure is a vertical sectional view of a tank permitting theelectrolytic refining of aluminum without involving any of the dangersand difllculties caused by the presence or the water jacket in apparatusof the usual type.

This apparatus includes a tank a, oi. circula shape, the inner wall ofwhich is merely made of bricks of magnesia b. The bottom of the tank isprovided with a sole 0, preferably plane, of compact carbon, connectedto the positive terminal of the electrolytic circuit. This sole servesto feed current to the anodic alloy d located above it.

The melted electrolyte e floats on the anode. It has been ascertainedthat this electrolyte does not attack the bricks of magnesia of the tankin which it lies, while it is known that such bricks are severelyattacked by the ordinary baths of fluoride utilized at temperaturesclose to 1000" C. As there is no cold wall, the whole of the electrolyteremains. in the melted state, its composition does not vary and there isno formation oi! scum on the anodic alloy.

The pure cathodic metal I floats on the electrolyte and current issupplied thereto through a carbon rod g or through any other suitablemeans.

It is not absolutely necessary to utilize very pure magnesia. It hasbeen possible to obtain very good results with a tank coated with veryordinary bricks oi magnesia, containing several percents of silica andiron oxide. After discharging a small amount of impiu'ities for somedays, the bricks no longer yield any impurity. It was possible to obtainin such a tank made of ordinary bricks of magnesia aluminum assaying99.995%.

As above stated the invention is not limited in any way to the specificexamples above given or to the apparatus above described, as changesmight be made thereto without departing from the principle of thepresent invention as comprehended within the scope of the appendedclaims.

What I claim is:

l. A method of refining aluminum which comprises superposing in anelectrolytic apparatus a layer of a molten alloy containing aluminum,forming the anode, a layer of melted electrolyte,

and a layer of aluminum forming the cathode, in which the electrolytecomprises 23 per cent in weight 01' aluminum fluoride, 17 per centsodium fluoride, and 60 per cent barium chloride.

2. A method of reflning aluminum which comprises superposing in anelectrolytic apparatus a layer of a molten alloy containing aluminum,forming the anode, a layer of melted electrolyte, and a layer ofaluminum forming the cathode, in which the electrolyte comprises 27 percent in weight of aluminum fluoride, 13 per cent sodium fluoride, and 60per cent barium chloride.

3. A method of electrically refining aluminum which comprisessuperposing in an electrolytic apparatus a layer of a molten alloycontaining aluminum forming the anode, a layer of melted electrolyte,and a layer of aluminum forming the cathode, in which the elctrolytecomprises a mixture of aluminum fluoride, an alkaline fluoride and analkali-earth chloride, the ratio of the amount of aluminum fluoride tothat of the alkaline fluoride being greater than the ratio of the amountof aluminum fluoride to that of sodium fluoride in cryolite, saidelectrolyte having a melting point between 700 C. and 800 C.

4. A method of electrically refining aluminum which comprisessuperposing in an electrolytic apparatus a layer of a molten alloycontaining aluminum forming the anode, a layer of melted electrolyte,and a layer of aluminum forming the cathode, in which the electrolytecomprises a mixture of aluminum fluoride, sodium fluoride and analkali-earth chloride, the ratio of the amount of aluminum fluoride tothat of sodium fluoride being greater than the corresponding ratio ofthe same bodies in cryolite, said electrolyte having a melting pointbetween 700 C. and 800 C.

5. A method of refining aluminum which comprises superposing in anelectrolytic apparatus a layer of a molten alloy containing aluminumforming the anode, a layer of melted electrolyte, and a layer ofaluminum forming the cathode, in which the electrolyte comprises amixture of aluminupi fluoride, sodium fluoride, and barium chloride, theratio of the amount of aluminum fluoride to that of sodium fluoride inthe electrolyte being greater than the corresponding ratio of the samebodies in cryolite, said electrolyte having a melting point between 700C. and 800 C.

ROBERT ANDRE GADEAU.

Nil

